doc: document the version subcommand

doc: document the version subcommand

Document the built-in version sub-command which displays
the OpenAFS version string. This sub-command is provided
by the cmd library.

Document the switch style -version option provided by the cmd
library for the initcmd based commands: afsmonitor, scout,
xstat_fs_test, and xstat_cm_test.

Change-Id: Id421d2c68a5c49a2b1a5abb2f3e9ca64ea36cd3e
Reviewed-on: http://gerrit.openafs.org/11161
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>